Iron Fluorescence in X-class Solar Flares

Nugget
Number:	512
1st Author:	Abhilash SARWADE
2nd Author:	et al.
Published:	December 8, 2025
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Introduction

Energetic solar flares originating in the Sun's million-degree outer atmosphere, the corona, emits a substantial flux of soft X-rays. While much of this radiation propagates into space, a fraction is directed downward, where it irradiates the cooler, visible solar photosphere.

This X-ray irradiation induces fluorescence in the photospheric neutral iron atoms, as shown in the sketch of Figure 1. Specifically, the neutral iron atoms absorb the incident soft X-rays and subsequently re-emit photons at a characteristic lower energy of 6.4 keV, producing the Fe K emission line. The resulting fluorescent signal from the photosphere serves as a powerful diagnostic, as its properties are intrinsically linked to the coronal flare occurring above it.

Figure 1: A schematic illustrating the process of X-ray fluorescence; here an Inner-shell electron is eajected by a photoelectric interaction from a soft X-ray continuum. Thereupon a line photon results from the subsequent re-filling of that state.

A primary theoretical prediction (Ref. [1]), is that the observed intensity of this signal should exhibit a strong dependence on the flare's position on the solar disk. A flare located near the disk's center is expected to produce a strong, readily detectable signal. Conversely, fluorescent emission from a flare occurring near the solar limb must traverse a longer, more oblique path through the dense solar atmosphere to reach an observer, resulting in more significant attenuation. This center-to-limb variation is a distinctive signature of photospheric fluorescence (Figure 2).

Figure 2: Schematic of the iron fluorescence process. (a) Limb Flare: When a flare occurs high in the solar atmosphere (black star), its X-rays illuminate a specific cap-like region on the solar surface (red area). This red patch in turn emits Fe Kα radiation (magenta arrow). (b) Disk Flare: When viewed face-on (here shown at an angle of 30°), the red patch on the solar disk is fully visible. The size of this illuminated area governs how much total fluorescence is produced and can reach the observer.

Instrumentation and Observations

The potential of this iron fluorescence as a diagnostic was recognized over 50 years ago. Pioneering observations with crystal spectrometers on missions like the Solar Maximum Mission (SMM) and Yohkoh provided the first glimpses of this phenomenon (Figure 3). However, these early spectrometers typically scanned narrow wavelength ranges and could not simultaneously measure the broadband exciting X-ray continuum.

Figure 3: Early observations of iron fluorescence. (Left) Spectra from SMM showing the resolved Fe Kα doublet (Ref. [3]). (Right) Observations from Yohkoh showing the Fe Kβ fluorescence line (Ref.[4]).

The Solar Low Energy X-ray Spectrometer (SoLEXS) on board India's Aditya-L1 mission is an instrument well-suited for studying this phenomenon. Throughout its first year of operations in 2024, a period corresponding to the maximum of Solar Cycle 25, SoLEXS recorded observations of 47 X-class solar flares.

A key capability of the SoLEXS instrument is its capacity to measure the entire soft X-ray spectrum (2-22 keV) simultaneously. This allows for the concurrent detection of both the high-energy coronal X-rays responsible for exciting the fluorescence (>7.11 keV) and the resultant Fe Kα emission (6.4 keV) from the photosphere. This instrumental design mitigates a significant source of uncertainty inherent in previous studies, which were often unable to perform simultaneous measurements of both the cause and effect.

SoLEXS obtained excellent data for a specific X1.0-class flare (SOL2024-06-21T08:48) located near the solar disk's center (θ ~ 29°), a geometry favorable for fluorescence detection. Ref. [2] describes these data in detail. At soft X-ray maximum, the standards flare X-ray spectral models fit extremely well, except near the locations of neutral Fe line features. The residual flux is thus the signature of the neutral Fe Kα line. Because the standard emission models (e.g., CHIANTI) only account for hot, ionized plasma in the corona, they do not include this emission from neutral iron. By adding a Gaussian component centered at 6.40 keV to the spectral model (as shown in Figure 4), this excess is fitted well. We can thus measure the fluorescence fluxes in the context of the flare's high-temperature emissions.

Figure 4: We reveal the fluorescence signature by combining the emission from the hot coronal plasma (green dashed line) with the photospheric iron fluorescence (blue dash-dot peaks at 6.40 and 7.06 keV), and the instrumental nickel fluorescence background (orange dotted peaks). The red line is the total model spectrum. By explicitly modeling the solar Fe Kα fluorescence at 6.40 keV alongside the instrumental Ni lines, the model achieves a better fit, resolving the discrepancy seen in the purely thermal fit.

Conclusion

The results presented in this study provide a validation of the photospheric iron fluorescence model using the unique capabilities of the Aditya-L1/SoLEXS instrument. The analysis of 47 X-class flares (Ref. [2], not reported in this Nugget), confirms that fluorescence is the dominant production mechanism for the observed 6.4 keV feature. SoLEXS observations provide direct measurements of both the irradiating flux and the fluorescent response. This work establishes the utility of modern, broadband Silicon Drift Detectors for quantitative studies of solar iron fluorescence, opening a new avenue for using the Fe K line to probe the geometry and physics of the flaring atmosphere.

Acknowledgments

Aditya-L1 is an observatory-class mission fully funded and operated by the Indian Space Research Organisation (ISRO). The SoLEXS instrument was designed and developed at the Space Astronomy Group of the U. R. Rao Satellite Centre (URSC), ISRO, with contributions from multiple entities within URSC. I thank the Aditya-L1 and SoLEXS teams for their dedication, and my co-authors for their significant contributions to the research presented here.

References

[1] "Iron Kα-fluorescence in solar flares: A probe of the photospheric iron abundance"

[2] "Iron Fluorescence in X-class Solar Flares: Aditya-L1/SoLEXS Observations"

[3] "SMM observations of K-alpha radiation from fluorescence of photospheric iron by solar flare X-rays"

[4] "Iron K beta Line Emission in Solar Flares Observed by YOHKOH and the Solar Abundance of Iron"